Method for making cellulosic board



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Nov. 25,, 1952 E. A. PATWM mm.

'METHOD FOR MAKING CELLULOSIC BOARD 4 Sheds-Shem 1 Filed Sept. 14, 1.950

Nov. 25, 1952 Filed Sept. 14, 1950 E. A. PAWUN ET AL METHOD FOR MAKINGCELLULOSIC BOARD zmwm Sheets-Sheet 2 Nov. 25, "21952 E. A. PATTON ETALZfimfiw METHOD FOR MAKING CELLULOSIC BOARD Filed Sept. 14, 1950 4:Sheets-Sheet 3 Nov. 25, 1952 E. A. PATTON mm.

ma ma METI-IOD FOR MAKING CELLULOSIC BOARD Filed Sept. 14, 1950 4Sheets-Sheet 4 UN [TED d'iATES Patented Nov. 25, 952

iowa, assignors to Curtis Companies incorporated, Clinton, liowa, acorporation of Iowa Application September 14, 1950, Serial No. 184,888

(Cl. ldti.5)

3 Claims. i This invention relates to a method for manufacturing acompressed wood board from granulated wood such as sawdust, finelysubdivided wood waste of the type obtained in mill work plants, and thelike.

Reference is made to the copending applica-' tions of Edward A. Patton,Merle W. Baker, Forrest F. Beil and Charles F. Curtis. I1, Serial No.28,158 (filed May 20, 1948, entitled Board of Compressed CelluloseMaterial and Method and Apparatus for Manufacturing the Same nowforfeited) and of Edward A. Patton and Forrest F. Beil Serial No.100,004 (filed June 18, 1949, and entitled Cellulosic Board and Methodof Treating the Same). These two applications show methods formanufacturing a compressed cellulosic board characterized by highstrength, freedom from warping and many other desirable characteristics.Reference is also made to the copending applications of Merle W. Baker,Forrest F. Bell, Charles F. Curtis, H, and Edward A. Patton Serial No.59,903 (filed November 13, 1948, and entitled Apparatus forManufacturing Boards of Compressed Cellulose Material and the Like nowU. S. Patent No. 2,583,249), Serial No. 59,902 (filed November 13,1948and entitled Pan Filling Machine) and Serial No. 117,634 (filedSeptember 24, 1949 and entitled Pan Filling Machine). These threeapplications show apparatus for practicing the methods of said firstmentioned two applications.

The apparatus and method of said five copending applications involvecompression molding of a mixture of granulated wood and a small amountof resinous binder in relatively shallow generally fiat pan or tray-likemolds. More particularly, the copending applications disclose themolding of mixtures of granulated wood with a resinous binder. Thesemixtures may contain as little as 4% resin and are characterized by amoisture content of at least 5%. The resinous binder is preferably, butnot necessarily, thermosetting and is characterized by a capacity forflowing under the temperature and pressure conditlons maintained duringthe molding operation for an appreciable period of time before the resinis set or cured or otherwise brought into the condition in which thebinder is present in the hi1- ished board. In the molding operation, apressure of at least 150 lbs. per square inch, but less than 500 lbs.per square inch is maintained, at least in the initial stage. Further,the exact pressure employed is correlated with the moisture content ofthe molding mixture as disclosed in detail herelnbelow. The temperatureis maintained for a sufliclent time to cure or set the resin. orotherwise bring the resin into the condition characteristic of thefinished board. Further, the margins of the layer of mixed resin andwood being molded are compressed to from 40% to of the thickness of theremaining portions of the compressed layer. Finally, the pressure isreleased slowly (within a time of several seconds or minutes), ratherthan all at once. The compressed edges may be trimmed on to leave apanel or board of uniform thickness.

The significance oi the above disclosed steps is explained as follows:

Since the margins of the layer being molded are compressed very muchmore than the remaining portions of the layer, the moisture content ofthe mixed wood and resin is maintained practically constant and uniformthroughout the molding operation. In other words, the compressed marginsor edges act as a seal to prevent the escape of moisture and themoisture content is kept uniform throughout the layer being molded.There is, therefore, no tendency to warping or curling after the moldingoperation has been completed due to uneven moisture loss with consequentgreater shrinkage of areas of relatively great moisture loss. Further,at a temperature of at least 280 F., a pressure of at least pounds persquare inch and a moisture content of at least 5%, and when the pressurehas been correlated with the moisture content as described hereinbelow,the wood particles are rendered plastic and flow so as to form a boardcharacterized by low porosity. h i strength and resistance againstchipping, in spite of the relatively small amounts of resinous binderpresent in the board. In this connection, it should be mentioned thatsince the resinous binder flows under the temperature and pressureconditions maintained during theirdtia'lstage of the molding operation,the resinous binder is distributed over the wood particles in a mannerthat utilizes more fully the binding properties of the resin. Finally,when the moisture content has been correlated with the pressure asdisclosed hereinbelow, there is little or no tendency to blister whenthe pressure is released slowly. Pressure must be maintained at leastlong enough to cause both the wood particles and the resin to flow intothe positions characteristic of the final board but the pressure may bereduced slowly and water vapor released, if desired, before the resin iscompletely cured as long as the moisture is retained for a suincientperiod to plasticize the wood particles for flow into their finalpositions. The pressure is strength,

kept below 500 lbs. per square inch, since otherwise the wood particleswill be at least partly hydrolyzed or changed chemically, as evidencedby darkening, lessened ability to absorb oil stains, and otherundesirable changes. Further, at pressures of 500 lbs. per square inchor higher, the boards tend to blister on release of pressure, even whensuch release is carried out slowly.

The board or sheet material prepared according to said method ischaracterized by high cohesiveness no tendency toward chipping or to thebreaking off of small particles, Particularly at edges), uniformphysical characteristics (strength, rigidity, and the like) from thecenter of the board or sheet all the way to the edge, freedom fromwarping, a tendency to swell at humidities higher than normal, it atall, principally in a direction normal to the plane of the board, ahygroscopicity no higher than ordinary wood, resistance against bending,ability to take paint and other finishes in the same manner as ordinarywood and a capacity for being sawed, nailed, screwed or planed.

When the boards prepared as disclosed hereinabove are removed from thepans or molds or trays in which they have been formed, the boards arehot (about 212 F.) and have a moisture content of about 2%. Aftercooling, and on standing exposed to the atmosphere, the boards absorbmoisture, finally reaching a moisture content that remains more or lessconstant. In other words, the moisture content of the boards reaches anequilibrium level which depends to some extent on the moisture contentof the surrounding atmosphere. At a, relative humidity of the atmosphereranging from 60 to 70%, the moisture content of the boards becomesconstant at the level ranging from 6 to 7% board moisture content.

Several days may be required for the compressed boards to reach theirequilibrium with respect to moisture content. During this time theboards undergo dimensional changes, for absorption of moisture isaccompanied by expansion of the boards. In other words, the boards reacha dimensional equilibrium at the same time as the board moisture contentreaches an equilibrium level. As a result, the boards, immediately aftercooling on removal from the pans or molds, tend to warp or to beotherwise distorted if then and there sawed into panels which at onceare incorporated with a cabinet or door or like structure havingsufficient rigidity to prevent free expansion or other dimensionalchanges in the panel made part of said structure. Of course, once theboards have reached their moisture and dimensional equilibria, theboards may be incorporated with doors, cabinets or like structures inthe form of panels having any desired dimensions, and will then not warpor be otherwise distorted.

The boards in question reach their equilibria with respect to moisturecontent and dimensions most rapidly if individually set on edge withtheir broad faces exposed to the atmosphere. If stacked with their broadfaces contacting, the boards require a much longer time to reach theirequilibria. But setting each board on edge even for a matter of hours ordays requires a considerable amount of space, equipment and labor. Onthe other hand, prolonged storage of stacks of boards requires asometimes inconvenient amount of space together with the necessity ofinspections for determining the condition of the stacked boards.

It is therefore an important object of the present invention to providea method for mak ing compressed boards from comminuted woodcharacterized by stability with respect to moisture content anddimensions so that said boards can immediately be incorporated withstructures such as doors and cabinet in which dimensional changes areprevented, the boards thereafter not being subject to warping ordistortion due to dimensional changes therein.

Another object of this invention is to provide a method for makingcompressed boards from comminuted wood in which the wood particles arenot changed or decomposed chemically so that the finished boards haveall the physical and chemical properties to be expected in merelycompressed wood particles including uniform light color and ability toabsorb oil stains uniformly.

Other and further objects and features will become apparent from thefollowing description and appended claims as illustrated by theaccompanying drawings showing, diagrammatically and by way of examples,apparatus and products according to this invention. More particularly:

Figure 1 is a diagrammatical side view of apparatus utilized foreffecting a complete process in the manufacture of boards fromgranulated wood;

Figure 2 is a. diagrammatic view of the hydraulic system of a hot pressforming part of the apparatus of Figure 1;

Figure 3 is a perspective view of a pan or tray used as a mold in theapparatus of Figure 1;

Figure 4 is an enlarged sectional view, with parts broken away, of thepan of Figure 3 with the material to becompressed therein, as it appearsprior to compression, with a cover disposed on top of the material;

Figure 5 is a view similar to Figure 4 but showing the parts as theyappear after the material has been compressed;

Figure 6 is an enlarged sectional view, with parts broken away, of thecompressed material after it has been removed from the pan;

Figure 7 is a view similar to Figure 6 but showing the finished boardwith the compressed or densified margins cut off, the cut off materialbeing shown in dotted lines;

Figure 8 is an enlarged perspective view of a water spray device formingpart of the apparatus of Fig. l

Figure 9 is a. cross sectional view taken along the line 9-9 of Figure8;

Figure 10 is a cross sectional view taken along the line Ill-l0 ofFigure 9;

Figure 11 is a cross sectional view taken along the line II-H of Figure9; and

Figure 12 is an enlarged fragmentary cross sectional view taken alongthe line |2-|2 of Figure 9.

Referring specifically to Figure 1 for a description of apparatus forpracticing the present invention, numeral H designates a conduit whichconveys, for instance, wood waste from a dust collector system in a woodmaking plant. The wood waste may contain a high percentage of knotsections, say, 50% knot sections and 50% machine waste.

The refuse or waste is delivered by the conduit H to a cyclone I2, whichis preferably equipped with a magnetic separator (not shown) to removeany metal therefrom, which may cause sparks and possibly a fire. Fromthe cyclone l2, the material is delivered through a conduit 13 toaeraera ii an ordinary commercial hammer-mill it, which pulverizes orgranulates the waste material and is equipped with a suitable screen notshown) to deliver pulverized waste directly into a storage bin 45. Thispart of the process is continuous, the remainder being accomplished by abatch method. The storage bin iii may be provided with an automaticshut-off device (not shown), which shuts off the delivery of wastethrough the conduit ll, when a predetermined level has been reached inthe bin l5. I

The pulverized material is fed through an outlet IG from the bin ill toa belt conveyor H. A screw conveyor i not shown) is provided in theoutlet conduit It, and the belt conveyor ill and screw conveyor aresynchronized electrically by any suitable means to introduce apredetermined amount of pulverized material into a waste measure I8. Anautomatic water valve (not shown) delivers a predetermined amount ofwater to each measured batch of pulverized material that is delivered toa muller or mixer iii. A predetermined amount of powdered or liquidresin or other binder is added to each measured batch of pulverizedmaterial. After mulling, this mixture is then dumped back onto a beltconveyor 24 through an outlet conduit 23, and is delivered to a hopper25 of the pan filling machine.

From the hopper 25, the pulverized and mixed material is delivered to abelt system, which is generally indicated by the numeral 25. The entirepan filling machine (which may be the machine shown in said copendingapplication Serial No. 117,634) is supported on a table it, and the panswhich are filled by the machine are shown generally at 28. A continuousbelt 29, and a second continuous belt 33, are provided for conveying thepulverized mixed material to the pans, and for conveying the pans to aloading rack 32, respectively. It will be noted that the loading rack isprovided with a number of shelves or supports 32a for the pans 28. Fromthe loading rack 32, the pans are delivered either by manual ormechanical means to a hot press generally indicated at 33. The hot pressitself is of modified standard design, and pressure is applied to thematerial in the pans, and at the same time the material is heated. Whenthe compression step is completed, the pans with the compressed ma{terial therein are delivered to the unloading rack 34, which likewisehas a number of shelves 36a for the reception of the pans 28. The pansare then removed from the rack fi l, either manually or mechanically,and the compressed material is taken out of the pans by inverting them.The inverted pans 28 are then placed on a gravity roller conveyor 35,which terminates adjacent the pan loading mechanism.

The compressed boards indicated at it; are placed on edge in a coolingraclr ti. After having been cooled, the boards are passed through aspray device generally indicated at 32 in which each board or panel ismoved between opposed sets of spray nozzles. After the boards or panelshave been sprayed with water, they are piled on a support to form astack M in which the boards are left for a few hours. During this timethe moisture distributes itself evenly through each of the boards orpanels which are then ready for trimming and sanding.

Referring now to Figures 3 through 7 for a detailed description or thepans 28 and the boards formed therein, the pan 28 is formed, preferably,of aluminum, because of its lightness and heat conductivity.Furthermore, the aluminum is of d fairly thick gauge and does not havetoo much tendency to warp under heat. Also. there is very littletendency for the compressed material to stick to the aluminum stu'face.Obviousjy, however, other metals may be used for the pans, such as brassor iron.

The pan 28, preferably, comprises a flat base member bu which has anangle shaped flange 5i secured thereto, as by rivets 52. The horizontalportion lid of the angle 5! overlies the base plate 5G, for a purposeherein described.

The mixed granulated material to be compressed is shown in Figure 4 bythe numeral 55. A cover 369 is suitably superimposed over the material55 and has its edges spaced from the upstanding portion of the angle 56.

When the compressed board lid (shown in Figures 6 and '7) has beenremoved from the pan 28, the board d ll includes a thin flange t5extending completely around the main portion of the board, being formedbetween the horizontal angle portion til and the opposed margins of thecover it. As shown in Figure 7, the thin flange or border 65 togetherwith a slightly upturned edge 36 (formed between the edge of the cover38 and the upstanding portion of the angle El) are removed to leave aboard of uniform thickness. The portions tlti and d6 thus removed, forinstance, by sawing, may then be returned to the waste or refusematerial and repulverized.

Referring specifically to Figure 2, a hydraulic system for operating ahot press is there shown. The hydraulic system is constructed alongconventional lines except for the inclusion of a needle throttling valve52 for a purpose to be described.

The hydraulic system comprises an oil supply tank 63; to which isconnected a high pressure pump 6d and a. high volume pump 65. A pipeline 66 is connected with the outlet of the high pressure pump and hastwo normally closed valves 67, 68 therein. A normally closed check valve69 is provided in an outlet pipe H from the high volume pump ed. A pipeline 72 is connected between the valves El and 58 to a cylinder drainvalve chamber i3. Likewise, pipe H5 is connected to a pipe l5 extendingbetween the valves '58 and 89 and pipe it is also connected to thecylinder drain valve chamber F3. The pipe is then connects the cylinderdrain valve chamber it with a hydraulic piston H which provides therequired pressure for the hot press 33.

The press 33 and the hydraulic system are provided with a standardelectrical timer (not shown), which maintains a high pressure on thehydraulic system until the pressing is completed. At this time, asolenoid operated valve 18 is partially opened. The needle valve 62functions as a pilot valve for the valve it and is adjusted so as toprovide a very small opening. The high pressure hydraulic fluid from thehydraulic piston ll slowly passes back through the pipe 76, through onebranch of the pipe l2, through the needle valve 62, through thepartially open solenoid operated valve l8 and back to the oil supplytank through a pipe l e. Since the valve 72 is so adjusted as to permitthe fluid to flow slowly into the main valve, a number of seconds Willelapse while the initial pressure is being reduced. As soon as the pilotvalve 62 is full of hydraulic fluid, the main valve '58 is openedcompletely and the press opens rapidly. A pipe 8% having a. hand valvetherein connects the cylinder drain valve chamber ill with the oilsupply tank 53 for the aeraera obvious purpose of draining the cylinderof the hydraulic piston 'I'i when desired.

Since there is considerable internal steam pressure or superheated waterin the panel during the pressing operation, the panel 40 is liable toblister or explode if the press is permitted to open instantaneously.With the needle valve 62 installed as shown and adjusted to a very smallopening, the high pressure oil is forced to pass through the smallopening when the solenoid valve I8 trips or opens. Therefore, severalseconds are required for the pressure to be reduced on the press 33,thus causing a gradual release of pressure in the board and eliminatingblistering and possible exploding of the pane1 40.

The operation of the apparatus and process has already been described upto the time that the mixed pulverized material is delivered to thehopper 25. The pan filling device preferably includes means formaintaining the granulated material in loose form. The pan fillingdevice fills the pans 28 with a layer or granulated material of uniformthickness, regardless of any warping or twisting of the pan. After thepans have been filled with the material to be compressed, the pans arecarried into the hot press 33, the covers having been placed over thepans so that they may be forced downwardly into each pan when proceedingaccording to the specific method illustrated in Figures 4 through 7.

Since the metal leg 53 cannot be compressed, the portion of the panelimmediately above it is compressed considerably more than the remainderof the panel, producing an extremely dense edge 45, 40 around the panel,which prevents the escape of an excessive amount of steam and moistureduring the pressing process. A seal results around the edge of thepanel, and in addition to preventing the escape of moisture, alsomaintains uniform moisture distribution throughout the panel duringpressing. Uniform moisture distribution brings about uniform physicalcharacteristics in the finished panel and, in particular, minimizesinternal strains in the panel, resulting in a flat panel with a minimumtendency to bow or curl due to internal strains resulting from unequaldistribution of moisture during pressing.

Similar results are obtained when proceeding according to the methodsillustrated in Figures through 17 in said copending application SerialNo. 59,903.

Another difiiculty encountered in retaining the steam in the panelduring the pressing operation is the tendency of the panel to explode,due to internal steam pressure, when the pressure on the hot press 33 isreleased. This has been overcome by installing a needle valve 82 in thehydraulic system of the hot press to permit very gradual release ofpressure on the panel, as explained above.

After the boards have been formed by compression in the hot press 33 andthereafter removed from the pans 28, as by inversion of the pans, thestill hot boards are placed on edge in the cooling rack M and thereexposed to the atmosphere. Ordinarily an exposure of from 20 to minutes(or not more than minutes) suffices to cool the boards down toapproximately 100 F. or at least to a temperature, say, of 120 F., wheremoisture not in excess or" 7% would not evaporate off rapidly. In otherwords, the panels or boards are cooled down to a temperature at whichthe final mixture equilibrium content (0 to 7%) can be maintainedthroughout the boards or panels. If the panels are sprayed while at 212F. (the temperature at which the panels are removed from the hot press),the panels warp. Such warping is apparently due to rapid evaporation ofthe applied moisture from the panel surfaces at this elevatedtemperature so that in fiat piling one panel surface is exposed to theair for a few seconds longer than the other surface and therefore has alower moisture content than the other surface. Such uneven distributionof moisture as between the two sides of the panel causes severe cuppingof the panel.

The spray device 42 for applying moisture to the panels is shown indetail in Figures 8 through 12. The device is supported by legs andincludes a relatively shallow longitudinally extending trough 94provided with a discharge conduit 00. Two sprocket wheels 88 rotatablydisposed in said trough near the ends thereof have a chain I00 trainedtherearound. The upper run of this chain moves over the bottom of achannel I02 extending above and parallel with the trough 94, having itsside walls extending above the chain I00. One of the wheels 98 isrotated by force, transmitted from a motor I04 through a chain I06 or byany other suitable means.

A rod I08 is supported by standards IIO above, slightly to one side ofand in parallelism with the channel I02. Thus, the edge of a panel 40may be placed on the chain I00 in the channel I02 and the upper marginof the panel will then be supported by the rod I08 as the panel is movedlongitudinally by the chain I00. At its discharge end, the rod i08 isbent horizontally, say, by 45, as indicated at 108a. to cause the boardsto tip over as they are discharged from the spray device.

The trough 04 includes a central upwardly divergent portion 94a formingthe bottom of a spray chamber having side walls I II and a roof H2. Theside walls III are each formed with a vertical recess IIIa accommodatinga vertically extending water pipe II4 supplied with water through aconduit I I6. A plurality of nozzles I I8 extend transversely into thespray chamber from each of the pipes H4. These nozzles are spacedprogressively further apart toward the bottom of the spray chamber. Ifdesired, each of the pipes H4 may be branched at the level of thehighest nozzle IIS, as shown at Illa, and this branch may terminate in anozzle II8a. Such a nozzle arrangement provides for the application ofmore water at the upper margin of each panel 40, to compensate for therun down of water over the sides of the panel.

The spray chamber is also provided with end walls I20 each formed with avertical inclined slot I22 through which the panels 40 enter and leavethe spray chamber. To prevent splashing of water through these slots,resilient flaps I24 may be aihxed to the outside of the walls I22 toproject over the slots I22, for instance, into wiping relationship withthe panels 40.

As explained hereinabove, the panels 40, after cooling in the racl: 4|,are passed through the spray device 42 and there moistened with water.The moistened panels are thereafter piled fiat as a stack 44 and leftthere until moisture and dimensional equilibrium has been reached, whichwill ordinarily be a matter of a few hours. The panels may then havetheir borders 45, 46 trimmed off and, after sanding, are then infinished condition.

The nozzles H8 form fan-shaped sprays directed onto both sides of thevertically disposed boards. The above described nozzle arrangement andthe fact that the chain I00 moves the boards slowly and at a constantslow speed through the aeiacis it spray chamber cause the boards to bemoistened uniformly. Any slight excess of water (above the amountrequired to raise the board moisture content to 6 or 7%) sprayed intothe boards is not absorbed but runs 017: the boards, when the boards aresprayed at about 100 F.

In this connection, it may be pointed out that the compressed boards,when removed hot from the press, contain approximately 2% moisture. Therest of the original moisture content is lost as steam when the press isopened. By adding about 4 to 5% moisture, the panels or boards arerendered perfectly stable dimensionally at arelative humidity of 60 to70% and dimensional changes at other relative humidities are reduced tosuch small values that they can be disregarded.

The following materials have been found to provide a very satisfactorypanel formed of compressed wood or other cellulose type material.Disintegrated wood of any species of tree may be used. Very satisfactoryresults have been obtained with pine wood. Preferably at least 50% ofthe wood is disintegrated to a 16 to 40 mesh particle size.

The resin employed is preferably one having a how point not higher than125 C. lhe resin may be a thermosetting resin capable of flowing for anappreciable period of time before it is cured or set in the press andcapable of acting as a bonding agent for the wood particles. We preferto use a resin having a curing time of from 40 to 100 seconds at 150 C.Resins of various chemical compositions share these characteristics. Wecan use, for instance, resins oi the phenolformaldehyde type or theurea-formaldehyde type, or iurfural resins and the like. Obviously,resins characterized by excessive tendency to absorb water or byinsufidcient resistance to weathering agents or having other undesirablecharacteristics should not be employed.

We have successfully used, inter alia, three phenolformaldehyde resinscharacterized by the following flow points and cure times:

It is understood that the thermosetting resins herein referred to arecapable of curing or'setting under the conditions of the moldingoperation. In other words, the binding agents employed may or may not beresinous when initially incorporated with the granulated wood but aredefinitely present as resins in the finished panels. We may thereforeemploy binding compositions made up of resin-forming materials in anyresinforming stage short of the final or cured or set stage. Theresinous binding agent may be employed in wet or dry condition. Weprefer to use a solid finely pulverized resin-forming composition, sincesuch products are most easily and most uniformly blended or mixed withthe wood particles. Nevertheless, we can also employ moist or dissolvedor dispersed resin-forming compositions, due regard then being bad forthe moisture content of the resin-forming composition when making up themixture to be molded.

The amount of resin employed may range upwardly from 4% to 5 of themixture being molded. We prefer to employ from. 5% to 8% resin. When adry powdered phenolfornialdehyde resin is used, very satisfactoryresults have been obtained at a resin content of from 6% to '7Blistering occurs at resin contents of about 14% or higher, for suchhigh resin contents apparently prevent the free escape of steam from theresulting dense boards when the press is opened. We prefer to keep theresin content at from 5% to 8%, to keep the cost at a minimum.Obviously, the exact amount of resin to be used will vary somewhataccording to the specific nature of the particular resin being used. Ingeneral, more resin is used when the wood is more finely disintegrated.

The water content of the molding mixture is maintained at from 5% to25%, depending on the pressure employed in the molding operation. Atlower moisture contents, the panels obtained are characterized byexcessive thickness, structural weakness, excessive porosity, thepresence of voids in the interior of the panel and by pitted surfaces,even when relatively high pressures are used. At moisture contents inexcess of 25% there is a tendency for the panels to stick or adhere tothe mold walls and to the formation of blisters or even to explosivedisintegration of the panel on release of the pressure, whether or notsuch release is accomplished slowly, if sufiicient pressure has beenused to form a firm panel. The correlation between the moisture contentand the pressure is discussed hereinbelow. Wood waste accumulated frommillwork operations commonly contains about 6 to 8% moisture. Thismoisture content is taken into account when the total moisture contentof the pressing mixture is calculated.

It should be understood that besides the above enumerated ingredients,other materials may also be incorporated with the molding mixture. Suchadded material may include pigments such as titanium dioxide, ironoxides and the like, inert fillers such as chalk or barium sulfate,materials commonly used as fillers or extenders for resins, finelydivided carbon and many other materials.

The above disclosed ingredients of the molding mixture are mixed witheach other at a temperature below the flow point of the resin.

The pressure applied during the hot molding operation ranges from 150 to400 pounds per square inch or higher but does not exceed 500 pounds persquare inch. The pressure is correlated with the moisture content of themolding mixture according to the following table:

I P i B d i cloisture rcssurc in cum s per roa ontent Square inch RangePreferred hmmmc Range Percent Percent 22-25 L. 17-23 20 12-18 15 7-l3 in5-8 7 l Pressure in Pounds For I 7 Square Inch Moisture Content ill 1 or(cut Minimum Maximum 150 200 Hill) 300 400 400 500 The exact pressuresand moisture contents to be employed will vary, within the tabulatedlimits, according to a number of factors such as the thickness, strengthand density required or desired in the finished panel. Obviously, thesecharacteristics vary according to the end use of the finished panel.Further, moisture contents and pressures will vary somewhat, within thetabulated limits, according to the nature and prior preparation of thewood, the nature and amount of specific resin employed, and likefactors. In making panels suitable for most, if not all purposes, on alarge scale, we prefer to use a molding mixture containing from 10 to15% moisture, and to press this mixture at from 300 to 400 pounds persquare inch, using a powdered phenol-formaldehyde resin as binding agentin an amount ranging from to 8%. Thus, a batch of material to be moldedmay have the following composition: 86.3% by weight pulverized millwaste 7.7% by weight water 6.0% by weight powdered phenol-formaldehyderesin having a how point of li0-125 C. and a cure time of 80-100 secondsat 150 C.

As explained hereinabove, the pressure is at least sufficient, at theprevailing moisture content and temperature, to cause the wood to beplasticized and at the same time not great enough to cause blisteringwhen the pressure is released slowly.

The temperature of molding is at least 280 or 300 F. A temperature Of338 F. insures very satisfactory results with the above disclosedspecific mixture. In general, the temperature must be sufficient tobring about curing or setting of any thermosetting resin employed. Thetime of molding should be sufiicient to bring about curing or setting atthe prevailing temperature. Ordinarily, from about 3 /2 to minutesmolding time is sufficient. With the above disclosed specific mixture, amolding time of 5 minutes has been found satisfactory. The full pressureshould be applied at the beginning of the molding operation, to insurefiow of resin before the resin is cured or set. When longer moldingtimes and higher temperatures are employed, the resulting panel materialwill be more stable dimensionally under varying humidity conditions. i.e. the material is less hygroscopic.

The pressure is applied for a period of time to compress the layer ofmolding mixture to its final dimensions. If desired, the full pressurecan be applied throughout the whole molding operation, although verygood results have also been obtained by slowly reducing the pressure toa lower value as soon as complete compression has been effected.

The molds may be coated with magnesium stearate to prevent adherence.For the same purpose, the molds may be preheated, say. to 150 to 175 F.before the press mixture is introduced.

In the molding operation, the margins on the layer being compressed arecompressed to about 40% to 60% of the thickness of the middle portionsof the finished panel. Some warping tendency is evident if the marginsare compressed to less than 60% of the thickness of the remainder of theboard. Wood cannot be compressed to lem than about one-third of itsoriginal thickness. Hence. when the edges or margins have beencompressed to about one-third of the thickness of the remaining portionsof the panel, these margins act as stops preventing further compressingof the middle of the panel.

Preferably, the margins are compressed to about 45% to 55% of thethickness of the middle portions of the panel. In the case of the abovedisclosed specific mixture, very satisfactory results have been obtainedby compressing the margins to one-half of the thickness of the remainingportions of the panel. In the case of a panel 4 ft. square, compressedmargins 1 wide function very satisfactorily to seal the moisture contentof the pressing mixture.

To prevent darkening, either of the whole panel (excluding the densifiedsealing edge or margin) or parts thereof, and to permit uniformabsorption of oil stains and the like, the pressing operation isconducted so that no significant decomposition or other chemical changesare efiected in the wood particles during the pressing step. For thispurpose, the molding pressure is kept below 500 pounds per square inchand the full molding pressure is applied for less than 10 minutes, atleast when the pressure ranges between 400 and 500 pounds per squareinch. At pressures below 400 pounds per square inch, the pressure may beapplied for longer periods than 10 minutes. Finally, the temperature isalso kept below levels causing discoloration of the wood and reducedstain absorption. More particularly, temperatures up to about 360 F. aresafe at pressures below 500 pounds per square inch applied for less than10 minutes. At pressures less than 400 pounds per square inch,temperatures higher than 360 F. may be used, say, up to 400 F. However,as long as the flow point of the resin-forming binder is exceeded byabout to F., no particular advantage is gained by further raising thetemperature.

To show the effects of various pressures applied for various times, thefollowing experiments are described. All these experiments were carriedout in the above described apparatus, the margins of the layer ofmaterial being molded being densified as described hereinabove.

A molding mixture consisting of 93% sawdust (dry basis) and 7% water wascompressed for 10 minutes at 345 F. and at a pressure of 500 pounds persquare inch and yielded a board having a strength barely sufiicient tohold the board together.

A second molding mixture consisting of 86% sawdust (dry basis), 7% waterand 7% of a resin binder was compressed for 10 minutes at 345 F. and ata pressure of 500 pounds per square inch and yielded a blistered boardof dark color.

A third molding mixture consisting of 87% sawdust (dry basis) 6% waterand 7% of a resin binder was compressed for 4 /2 minutes at 320 F. andat a pressure of 480 pounds per square inch and yielded a board ofuniformly light color devoid of blisters.

A fourth molding mixture consisting of 86% sawdust (dry basis), 7% waterand 7% resin binder was compressed for '10 minutes at 345 F. and at apressure of 480 pounds per square inch and yielded a board having a darkcentral portion and a light colored marginal portion. The centralportion absorbed less oil stain than the lighter marginal portion.

It should be noted that the above disclosed restrictions as to pressureand time apply only to the conditions under which the present method iscarried out, i. e., where the margin of the layer of granulatedwood-resin mixture is densified to form a seal retaining moisture withinthe molding mixture inside said densified margin. In this case, themargin is densified to almost or about its!) the limit of itscompressibility, while this is not true of the material inside saidmargin. The latter material is not densified as much as the margin. Thedensified margin (which is subsequently trimmed off) is usually darkenedand characterized by reduced capacity for absorbing oil stains or thelike, as compared with the material within the margin. Thus, the step ofcompressing the margins more than the rest of the layer being moldedserves not only to form a seal against the escape of moisture but theheavily densified margin material also serves as a stop preventingsimilar heavy compression of the material within the margin whichotherwise would be darkened and have its ability to absorb oil stainsreduced. In other words, the heavy densification of the marginalmaterial permits the applications of relatively high over-allcompressing forces which would cause over-all darkening and otherundesirable changes in the compressed board in the absence of suchmarginal densification.

The edge sealing discussed in the preceding paragraph furthercontributes to uniformity in strength throughout the compressed boards,as illustrated by the following experiment. A number of souare boards or2 foot width were prepared using a sawdust mixture containing about 7%resin binder, as tabulated:

Boards Nos. 1 and 2 were about equally strong. Both these boards showeda gradual slight increase in strength inwardly from the edge for adistance of or 6 inches and were then fairly uniform in strength all theway to the center.

Boards Nos. 3 and 4 were quite weak over a marginal area 2 inches wide.At a line extending 2 inches from the edge, there was an abrupt increasein strength. From this line inwardly, the strength increased graduallyto a very high value at the center of the board.

From the foregoing it will be apparent that we have provided an improvedmethod and apparatus for forming dense board from granulated orpulverized wood or other cellulose material.

The panels or boards prepared as disclosed hereinabove are made up ofwood that has not been modified chemically to a significant extent andof resin in an amount of, say, from 6 to 7%. The boards will have aboutthe same hygroscopic characteristics (tendency to absorb water) as thewood from which the boards have been prepared. The color of the boardsis approximately the same as the wood contained therein. It should benoted, in this connection, that the color of the boards is uniform anddoes not vary locally, as contrasted to the different colors of the sapwood and heart wood oi pine and to the local color variations inplywood. The boards prepared from a pressing mixture containing from 10%to moisture at from 300 to 400 pounds pressure are characterized bymoduli of rupture in static bending of from 4000 to 5000 pounds persquare inch in all directions. With respect to strength in staticbending, these boards are one-half as strong as solid wood. one-half asstrong as 3-ply plywood with surface plies running in the longdirection, and twice as strong as B-ply plywood with surface pliesrunning cross-wise. As to impact resistance these boards comparefavorably with inch pine plywood or fir wood of equal thickness or withsolid wood of equal thickness, and greatly exceed many conventionalbuilding boards. For comparison with the latter, our boards aredistinguished by firmer edges that will not splinter like plywood nordent as easily as plywood or solid lumber when the edges are treatedroughly. The surface of our boards resist denting many times better thanpine plywood or solid lumber. Our boards shrink or swell but little. Forinstance, a panel of 3-foot width will swell or shrink only about inchwith a moisture change of 6%, while a pine plywood panel will swell orshrink inch and pine lumber panel will shrink or swell l;- inch. Ourboards are superior to plywood in resistance against warping and not asliabale to damage on subjection to elevated temperatures. Our boardshave surfaces-excellently adapted to receive a finish, such as paint,being more absorptive so that the paint will be more firmly bondedthereto, and the paint coats do not show the hair line checks typical ofpainted veneers, and due to al ternate transverse swelling andcontraction of oriented cellulosic fibers. The initial paint coatapplied to our boards yields a finish similar to painted metal. Thesurface of our boards accepts readily any color stain and the stain willnot bring out any local color variations, as is the case with lumber orplywood. The boards may be treated to simulate natural wood grain andthe staining characteristics of natural wood by the methods disclosed insaid copending application Serial No. 100,004. Our boards are easilymachined, with any wood-working machinery, and can consistently beproduced with any desired hardness, color, size or othercharacteristics.

Changes in composition and procedure may be made without departing fromthe real spirit and purpose of our invention, and it is our intention tocover by our claims any modified forms which may be reasonably includedwithin their scope without sacrificing any of the advantages thereof.

We claim as our invention:

1. In a method of preparing a cellulosic board which comprises providinga mixture comprising mechanically disintegrated wood and a resinformingbinder in an amount ranging from about 4% to about 14% by weight of saidmixture, said mixture having a moisture content ranging from about 5% toabout 25%, subjecting a layer of said mixture at an elevated temperatureranging from about 280 to 400 F. to a pressure ranging from about toabout 500 pounds per square inch for at least 3 but less than 10minutes, said pressure being correlated within said range with lthtemoisture content of said mixture as tabua ed:

' Pressure in Pounds Per said elevated temperature being maintainedbelow 360 F. whenever said pressure amounts to at least 400 pounds persquare inch, the margins of said layer being compressed more than theremainder of said layer to 'seal said remainder against moisture lossduring the pressing operation, and thereafter slowly releasing saidpressure to prevent blistering of the compressed board, the board thusprepared having a moisture content of less than 6%, the improvementcomprising exposing the compressed board to the atmosphere for coolingthe board at least down to 120 F. and thereafter moistening the boardwith water in an amount sufficient to raise the moisture content of saidboard at least to 6%.

2. In a method of preparing a cellulosic board which comprises providinga mixture comprising mechanically disintegrated wood and a resinformingbinder in an amount ranging from about 4% to about 14% by weight of saidmixture, said mixture having a moisture content ranging from about 10%to about 25%, subjecting a layer of said mixture at an elevatedtemperature ranging from about 280'" to 400 F. to a pressure rangingfrom about 150 to about 400 pounds per square inch for at least 3 butless than 10 minutes, said pressure being correlated within said rangewith the moisture content of said mixture as tabulated:

Pressure in Pounds Per Square Inch Minimum Maximum Moisture Content inil-rccm 150 a lo-QO "i 200 i 300 10-15 300 i 400 id to F. and thereaftermoistening the board with water in an amount sufficient to raise themoisture content of said board at least to 6%.

3. In a method of preparing a cellulosic board which comprises providinga mixture comprising mechanically disintegrated wood and a resinformingbinder in an amount ranging from about 4% to about 14% by weight of saidmixture, said mixture having a moisture content ranging from about 10%to about 15%, subjecting a layer of said mixture to a temperature offrom about 280 to 400 F. and a pressure of about 300 to 400 pounds persquare inch for at least 3 /2 but less than 10 minutes, the margins ofsaid layer being compressed more than the remainder of said layer toseal said remainder against moisture loss during the pressing operation,and thereafter slowly releasing said pressure to prevent blistering ofthe compressed board, the board thus prepared having a moisture contentof less than 6%, the improvement comprising exposing the compressedboard to the atmosphere for cooling the board at least down to 120 F.and thereafter moistem'ng the board with water in an amount suflicientto raise the moisture content of said board at least to 6%.

EDWARD A. PA'I'ION.

FORREST F. BEIL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 667,385 Bright Feb. 5, 19011,900,166 Dix Mar. 7, 1933 2,033,411 Carson Mar. 10, 1936 2,044,213Irvine June 16, 1936 2,214,641 Massey et al Sept. 10, 1940 2,314,797Morris et a1 Mar. 23, 1943 2,348,081 Linzell May 2, 1944 2,402,554Irvine et al June 25, 1946 2,437,492 Allen Mar. 9, 1948 2,480,851 GossSept. 6, 1949

1. IN A METHOD OF PREPARING A CELLULOSIC BOARD WHICH COMPRISES PROVIDINGA MIXTURE COMPRISING MECHANICALLY DISINTEGRATED WOOD AND A RESINFORMINGBINDER IN AN AMOUNT RANGING FROM ABOUT 4% TO ABOUT 14% BY WEIGHT OF SAIDMIXTURE, SAID MIXTURE HAVING A MOISTURE CONTENT RANGING FROM ABOUT 5% TOABOUT 25%, SUBJECTING A LAYER OF SAID MIXTURE AT AN ELEVATED TEMPERATURERANGING FROM ABOUT 280* TO 400* F. TO A PRESSURE RANGING FROM ABOUT 150TO ABOUT 500 POUNDS PER SQUARE INCH FOR AT LEAST 3 1/2 BUT LESS THAN 10MINUTES, SAID PRESSURE BEING CORRELATED WITHIN SAID RANGE WITH THEMOISTURE CONTENT OF SAID MIXTURE AS TABULATED: